1. Field of the Invention
The present invention relates to a color picture tube. In particular, the present invention relates to a color picture tube in which a radius of curvature of a panel outer surface is 10,000 mm or more.
2. Description of Related Art
In general, as shown in FIG. 3, a color picture tube includes a vacuum envelope 9 composed of a panel 1 having a substantially rectangular useful portion 1a and a skirt portion 1b connected to the periphery of the useful portion 1a, and a funnel 2 in a funnel shape connected to the skirt portion 1a. On an inner surface of the useful portion 1a of the panel 1, a phosphor screen 3 is formed, which is composed of black non-light-emitting material layers and three-color phosphor layers provided in regions where the black non-light-emitting material layers are not formed. A shadow mask 4 is opposed to the phosphor screen 3. The shadow mask 4 is held on a mask frame 11 in a rectangular frame shape, and the mask frame 11 is attached to an inner wall surface of the panel 1. In a neck 5 of the funnel 2, an electron gun 7 emitting three electron beams 6B, 6G, and 6R is provided. On an inner side of a large diameter portion of the funnel 2, an internal magnetic shield 10 attached to the mask frame 11 is placed. A deflection apparatus 8 is provided on an outer side of the funnel 2. The three electron beams 6B, 6G, and 6R emitted from the electron gun 7 are deflected by a magnetic field generated by the deflection apparatus 8, and pass through electron beam passage apertures formed in the shadow mask 4 to scan the phosphor screen 3 in horizontal and vertical directions, whereby a color image is displayed.
In such a color picture tube, in order to display an image without color displacement on the phosphor screen 3, it is necessary that the three electron beams 6B, 6G, and 6R having passed through the electron beam passage apertures formed in the shadow mask 4 should land correctly on the three-color phosphor layers. For this purpose, the relationship of the shadow mask 4 with respect to the panel 1 is important. Above all, it is necessary that an interval (q value) between the inner surface of the useful portion 1a of the panel 1 and a region (perforated region) of the shadow mask 4 in which the electron beam passage apertures are formed is within a predetermined allowable range.
Of all the electron beams emitted from the electron gun 7, only a part thereof reaches the phosphor screen 3. The remaining electron beams strike the shadow mask 4. At this time, the kinetic energy of the electron beams changes to thermal energy to heat the shadow mask 4. Therefore, the shadow mask expands thermally in accordance with the coefficient of thermal expansion of the material thereof, and its shape changes. Consequently, the positions of the electron beam passage apertures with respect to phosphors change, and when the change amount of these positions exceeds an allowable value, the electron beams cannot strike desired phosphors so that so-called mislanding occurs, which degrades the color purity of a display image.
In the thermal expansion of the shadow mask 4 caused by the irradiation with electron beams, in the case where only a part of the perforated region is irradiated with a large amount of electron beams, the change amount of the positions of the electron beam passage apertures with respect to the phosphors becomes particularly large, and the color purity is degraded significantly due to the mislanding of the electron beams. For example, as shown in FIG. 4, the following is known generally. In the case where only band-shaped regions 20 each extending in a minor axis (Y-axis) direction, positioned substantially at an intermediate portion between a screen center and a major axis (X-axis) end of a screen, are set to be a white display, and a region 21 other than the band-shaped regions 20 is set to be a black display, the color purity is most likely to be degraded. In the case of performing such a display, the perforated region of the shadow mask 4 deforms thermally as shown in FIG. 5. More specifically, the temperature of portions corresponding to the band-shaped regions 20, in which a white display is performed, in the perforated region increases locally, and these portions deform so as to protrude to a phosphor screen side (doming). When such local doming occurs, on the major axis where the movement amount in a tube axis direction of the surface of the perforated region becomes large, the color purity is degraded most significantly.
Recently, in order to enhance the visibility of a color picture tube, there is a demand that the radius of curvature of the outer surface of the useful portion 1a of the panel 1 is increased so as to bring the outer surface close to a flat surface. In this case, in terms of the strength of the vacuum envelope 9 with respect to the atmospheric pressure and visibility, it is necessary to increase the radius of curvature of the inner surface of the useful portion 1a. In order to obtain appropriate electron beam landing in accordance with the increase in the radius of curvature of the inner surface of the useful portion 1a, it is necessary to increase the radius of curvature of the perforated region of the shadow mask 4. However, when the radius of curvature of the perforated region of the shadow mask 4 is increased, the change amount of the positions of the electron beam passage apertures with respect to the phosphors due to doming increases, and the mislanding amount of the electron beams increases, so that the color purity is degraded significantly.
Therefore, in a color picture tube having the panel 1 with a substantially flat outer surface, in order to suppress doming, in most cases, an alloy mainly containing iron and nickel, having a low coefficient of thermal expansion, is used as a material for the shadow mask 4. For example, a 36 Ni Invar alloy or the like is used frequently. In this case, the iron-nickel alloy entails high cost, while providing a coefficient of thermal expansion of 1 to 2×10−6 at 0° C. to 100° C., which is effective for suppressing doming. Furthermore, the iron-nickel alloy has large elasticity after annealing, so that it is difficult to form a curved surface from such an alloy by press forming and to obtain a desired curved surface. Even if the iron-nickel alloy is annealed, for example, at a high temperature of 900° C., the yield point strength is about 28×107 N/m2. Thus, it is necessary to treat the alloy at a considerably high temperature in order to set the yield point strength to be 20×107 N/m2 or less at which press forming generally is considered to be easy. Particularly, in a color picture tube with a flat panel outer surface, the radius of curvature of the perforated region of the shadow mask is large, so that press forming is further difficult.
In the case where press forming is insufficient, and undesired stress remains in the shadow mask 4 after press forming, the residual stress changes the shape of the shadow mask 4 in the course of production of the color picture tube, which leads to the mislanding of the electron beams, resulting in significant degradation in the color purity.
On the other hand, with aluminum killed steel mainly containing high-purity iron, the yield point strength can be set to be 20×107 N/m2 or less by annealing at about 800° C., so that press forming is very easy. Thus, regarding the aluminum killed steel, it is not necessary to keep the press die temperature to be high in the course of press forming, which is required in an Invar alloy, and the productivity also is satisfactory.
However, the coefficient of thermal expansion of the aluminum killed steel is high (i.e., about 12×10−6 at 0° C. to 100° C.), which is disadvantageous for doming. Particularly, in the case of applying the aluminum killed steel to a color picture tube in which the outer surface of the useful portion 1a of the panel 1 is substantially flat, there arises a serious problem such as the significant degradation in color purity.
JP 10(1998)-199436 A discloses a shadow mask in the shape of a substantially cylindrical surface, in which the radius of curvature in a major axis direction is almost infinite, and the radius of curvature in a minor axis direction is almost constant irrespective of the position in the major axis direction. Even such a shadow mask has an effect of suppressing doming to some degree. However, in the case of using an inexpensive iron material, a sufficient effect cannot be obtained. Furthermore, there is a problem that the weight of a panel increases.
As described above, in the color picture tube, when the radius of curvature of the outer surface of the useful portion of the panel is increased so as to enhance visibility, and the radius of curvature of the perforated region of the shadow mask is increased in accordance with the increase in the radius of curvature of the outer surface of the useful portion, the mislanding amount of the electron beams increases due to the thermal expansion of the shadow mask, and consequently, the color purity is degraded significantly.
Furthermore, in the case of using an iron material that is inexpensive and has satisfactory formability as a material for the shadow mask, the mislanding amount of the electron beams caused by the thermal expansion of the shadow mask further increases due to its large coefficient of thermal expansion, and consequently, the color purity is degraded significantly.